Depletion of forest cover and pollution of the environment are two major concerns of mankind today. Thus there is great impetus for developing technologies that prevent the cutting down of forests, as well developing “green technologies” that reduce the quantity of chemicals used in a process. For example, the use of steam for hydrolysis reactions would be preferable to the use of chemicals for achieving the same purpose.
Agricultural wastes such as sugarcane bagasse, grain and cereal straws, jute sticks, cotton stalks, etc. are fibrous cellulosic materials consisting of cellulose, hemicellulose and lignin in nearly the same proportion as most woods, and therefore if new technologies replace wood with these materials, they would help preserve many forests and the environment. Additionally, if all the components of such cellulosic fibres are separated into their pure component polymers, i.e., cellulose, hemicellulose, and lignin, then they could provide us with pure value-added organic raw materials useful in the production of a wide variety of industrial and consumer goods such as plastics, chemicals, raw material for biofuels like butanol and ethanol. This would cultimate into a biorefinery similar to a petroleum refinery etc. It is estimated that the annual availability of agricultural cellulosic fibre waste of all types of agricultural produce is around twenty billion tonnes. Considering the immense potential of this huge and largely untapped raw material, that are currently of little industrial use, there is great interest in developing new process technologies which will create value out of waste agricultural residues.
The worldwide demand for paper is so great that most fibrous plant based lignocellulosics matter have been investigated for pulping by various processes to produce paper grade cellulose pulp. This is achieved by removing the lignin and hemicellulose constitutents in ways designed to minimize loss and damage to the cellulose fibres. In general, these processes consist of cooking or digesting the lignocellulosic plant matter with pulping chemicals at elevated temperatures and times scales sufficient to cause an acceptable degree of delignification, so as to produce a cellulosic pulp of specified characteristics, especially color, strength, gloss and printability. In most processes a large fraction of the lignin and hemicellulose constituents extracted out from the lignocellulose plant material (“black liquor”) are a source of environmental pollution as the “waste liquids” in the form of “black liquor” are let out into waste streams after expensive anti-pollution treatments. This results in valuable industrially useful organic polymeric materials like lignin and hemicellulose being lost, which if separated, could have many industrial applications. Also, the cost of pollution abatements would be avoided.
Therefore, in recent years there has been seen great interest in developing clean fractionation processes for lignocellulosic plant materials not only for paper and pulp but also to obtain pure individual streams of cellulose, hemicellulose, and lignin for use as biorefinery.
To date, however, no simple and economically viable methods are known for efficiently separating purified fractions of lignin, hemicellulose and cellulose from lignocellulosic biomass, due to the complexity of the structure and ingredients present in the natural biomass materials. Inspite of this, much research is being done worldwide to find new processes to achieve these ends, as is evident from very large number of research publications and patents.
The production of high value plastics such as cellulose esters and food grade cellulose derivatives requires high quality cellulose raw material. Hitherto, cotton linters was the raw material of choice for the production of high quality cellulose based plastics and other common derivatives like cellulose ethers. However, cotton linters are expensive and their production is limited. In recent years, extensively purified wood pulp has also become available for the manufacture of plastics like cellulose esters. However, this wood pulp is becoming increasingly expensive, and the effects of de-forestation in several countries has made it imperative to seek use of alternate sources of renewable materials.
Cellulose pulp which is available at reasonable cost from pulp and paper mills generally pertains to paper-grade cellulose, which has an α-cellulose content in the region 75-84%, and significant quantities of hemicellulose and lignin can be tolerated and are indeed desirable in their application as printing paper. This relatively abundant cellulose pulp cannot be used for preparing high quality plastics like cellulose esters. The “dissolving grade” pulp available in the market place refers to wood derived cellulose pulp which is extensively purified to remove traces of lignin and hemicellulose to obtain high α-cellulose (above 94% α-cellulose content) which is then reacted with carbon disulfide to produce soluble cellulose xanthate. The latter is then spun into fibers (viscose rayon) with evolution of carbon disulfide and regeneration of cellulose. However, there is a need to develop a process which would avoid the use of wood which is scarce in many parts of the world, and replace it with annually replenishable cellulose containing agricultural wastes such as sugarcane bagasse, wheat straw, and other such materials, which is available in abundance each year.
Sugarcane bagasse is an agricultural by-product which has the general composition of 40-45% cellulose, 28-30% hemicellulose, and 19-21% lignin. The composition varies from geographical location and age of the plant. The pith content is in the range of 30-35%. For Brazilian and Egyptian bagasse, it has been shown that depithing before acid prehydrolysis produced higher yields and quality of cellulose. Cellulose from the bagasse fibre is known to have a higher degree of polymerization than the cellulose obtained from bagasse pith.
It is well known that efficient fractionation of polymeric constituents of plant biomass has been a major obstacle in economic and ecologically viable processes. A very recent paper explains a method of removing hemicellulose and lignin by hydrothermal treatment. However, no fractionation of pure components was possible by this method. Thus there is great incentive to develop this lignocellulosic material as a source for cellulose as well as associated polymers like xylan and lignin for economic gain as well as for preventing the denudation of the forests and protecting the environment.
Reference may be made to a recent paper, Andrew, D'Agostino, Davis, DuPlooy and Kerr, Pulping Conf. Vol. 2, 1053-1061. TAPPI Press: Atlanta, Ga. (USA) (1998) CA 130:169723, which explain a steam explosion method for fine paper production; however, no mention is made of separating lignin and hemicellulose.
Similarly, reference may be made to Faria et al. Cellul. Chem. Technol. 32(5-6), 441-455 (1998) CA 131:75098, which describe a steam explosion technique only for obtaining pure grades of cellulose but no mention is made of separating lignin and hemicellulose. A paper by Ibrahim and Glasser, Bioresource Technol, 70, 181-192, 1999 describes steam explosion conditions to separate both lignin and hemicellulose, and he found that at low severities of steam up to 72% of hemicellulose could be removed, and at high severities 82% of lignin could be removed. This objective and study of the work is very different from our process, wherein the objective is to remove only one component (hemicellulose) by steam explosion, and the subsequent step (alkali treatment) to remove the lignin component.
Kaar et al, Biomass and Bioenergy, 14(3), 277-287 (1998) studied steam explosion of sugarcane bagasse as a pretreatment for ethanol. Sawada et al., J. Chem. Technol. Biotechnol. 76(2), 139-146 (2001) were interested in steam explosion for maximizing the delignification of plant biomass.
Patent BR 9005762-A (Derwent Primary Accession No. 1992-269093[33]) describes a process for cellulose production from vegetable residues using steam cracking, water extraction, alkali delignification and bleaching. However, the effects of pith content were not evaluated to improve the quality of the cellulose.
Canadian Patent CA 1282777 describes a steam explosion process to dissociate and exhort lignin and optionally the xylan from the primary wall and middle lamella while retaining the structural integrity of the fiber bundle (52 layer) in order to get a material for diaper and similar absorbent material. However, there is no claim as to the applicability of these fibers for production of cellulose acetate plastics, cellulose ethers etc.
Canadian Patent CA 1217765 uses a steam explosion process as a pretreatment to make the substrate easy to treat by enzymes for conversion to sugar and alcohols, and for the solvent extraction of liqnin. This patent does not seem to isolate pure cellulose for use in value added cellulose esters and ethers.
U.S. Pat. No. 6,419,788 has the objective of using a steam explosion process to produce cellulose substantially free of liqnin, hemicellulose and extractives, and also to lower the cost of the process. The primary focus of this research appears to be an energy recuperations for economic viability, and uses only steam, water and oxygen in the process. In several other (U.S. Pat. Nos. 5,125,977; 5,424,417; 5,503,996; 5,705,369 and 6,022,419) the focus appears to use acids solutions so as to achieve hydrolysis rather than obtain highly pure cellulose.
Rauschenberg et al., Polym. Preprints (ACS), 31(1), 650-2 (1990) studied isolation of xylan oligomers from steam exploded biomass. Schultz et al J. Agric. Food Chem., 32(5), 1166-72 (1984) used steam exploded biomass for studying enzymic rate of hydrolysis. Thus, all these published studies are not directed towards obtaining pure fractions of cellulose, hemicellulose and lignin, all in high yields, which is the objective of our work as described in the process.